The present invention relates generally to semiconductor wafer processing for integrated circuits, and more particularly to magnetic sensing robotics for collision avoidance in semiconductor wafer processing systems, or other chemical wet stations.
The manufacturer of integrated circuits is becoming a highly automated process. In the semiconductor chip fabrication process it is often necessary to transport cassettes filled with wafers to and from several different processing machines within a particular environment. Depending on the types of chips being manufactured, the number and order of machines will vary. Thus, a fixed transportation system, for example, a continuous belt system, may be impractical. Rather, a preferred mode of wafer cassette transport from one process station to a different process station is through the use of a robotic arm which automatically transports a carrier containing semiconductor wafers from one processing station to a different processing station. On the end of the arm is a "hand". The arm and hand movements are controlled by a servosystem which directs the hand to accomplish any desired pattern of movements. Through programming the robot is directed through various movements in a desired sequence. The use of a sensor input provides the robot with a knowledge of the changes to its environment, i.e. intrusions. Robot control algorithms may monitor sensor states to make decisions concerning the suitability of certain actions.
Semiconductor wafers are processed with wafers held in carriers or boats containing, for example, 25 wafers each. Processing the wafers requires handling and manipulation of the wafer boats for transporting the wafers between process steps and loading wafer boats into process machines. The process machines may produce corrosive fumes and liquids which may contact and attack any handling surface. Additionally, the temperatures of liquids contained within these process machines may range from 20.degree. to 250.degree. C., with the IPA dryer temperatures being the highest. For these reasons, only certain materials may be used for holding and transporting the wafers from one processing station to another.
The wafers are extremely fragile and may be of very high monetary value. For this reason, handling techniques must be very reliable and done in a manner to protect the wafer carriers from being dropped, or forcefully crushed. When handling tasks are given to robots, the robot must be given the ability to sense and judge the safety of its load.
As wafer handling becomes more automated, robots are continually being introduced to manipulate the carriers through the process steps. Robotic handling of wafer carriers is relatively new and suffers from the inability to determine whether the carrier grasped by the robot arm is successful and the inability to detect a collision during motion of wafer carriers. Detecting collisions during motion affords a peripheral benefit of improving the safety of people and adjacent equipment which may be damaged by a non-sensing robot.
Typically, grippers available from robot arm manufacturers are successful in grasping a wafer carrier that is properly presented and which does not exhibit wear or distortion. This works well if the wafer carriers are accurately placed and the grasped carrier remains securely in the gripper. In practice, however, manipulation may be unreliable, limited by carrier distortion and accuracy limitations of the machine delivering the wafer carrier. When delivery deviations occur beyond the acceptance limits of the gripper, it may either miss the wafer carrier entirely, causing the downstream process machine to deal with an error condition, or the gripper may make a partial grip, causing distortion or damage to the wafer carrier, crushed wafers or a dropped wafer carrier.
Similarly, wafer carriers which are worn or otherwise distorted from original design dimensions may cause mis-grip problems when a non-sensing gripper is utilized. As above, the results are distortion or damage to the wafer carrier when the carrier slips out during movements. The carrier is also subject to entanglement at the pick-up position and may encounter foreign objects in its path of motion. Also, mis-programming or hardware failure could cause a collision path to be followed. In these cases, the carrier may be wrenched from the gripper or the carrier may be crushed between the gripper and the obstacle.
Although automated processing of semiconductor wafers is usually trouble free, a number of situations may arise in which collisions occur between one carrier and another carrier, with result in destruction of the carrier and/or the plurality of semiconductor wafers. For example, an error in the software controlling the automated process may lead to a collision, damaging the carriers and/or wafers. Alternatively, human error may be introduced which leads to collisions that damage or destroy portions of the carriers/wafers. For example, a user operating the automatic wafer processing system may bypass a particular automated feature, choosing to process the wafers manually at a particular station. If the computer system is not aware of the manual processing occurring at that station, collisions may occur when the automated process subsequently attempts to utilize the station.
One objective of the present invention, therefore, is to provide a collision avoidance technique for automatic wafer processing systems which prevents robotic crashes that destroy processing product such as carriers, boats, and/or semiconductor wafers. Currently, in conventional wafer processing systems, fuses and amplifiers are used to shut down the robotic arm when overcurrent in the robotic arm is detected. One disadvantage to this approach is that this type of safety feature functions only to protect the robot arm itself. Thus, for example, the robotic arm can meet with a relatively large amount of resistance, while not reaching its overcurrent limit, and still continue processing operations. This, in turn, may result in the damaging of wafer carriers and/or the wafers themselves. The damaged equipment not only is expensive to replace, but also results in a major cleanup effort having to be undertaken in order to return the processing station to its "clean room" state. Additionally, such collisions also lead to equipment "down" time, which is undesirable.
Several patents in the past have attempted to incorporate safety features to avoid robotic collisions. For example, U.S. Pat. No. 4,816,732 to Willson discloses a robotic hand for transporting semiconductor wafers. The robotic hand includes impact sensors which detect any amount of impact force encountered which is in excess of the amount tolerated by design. The impact sensors signal the robot mechanism to stop when an undesirable resistive or impact force is encountered. However, the mechanical nature of impact sensors, with tensioning springs, is a very dirty mechanism and not appropriate for cleanroom use. Additionally, such sensors are unable to prevent collisions between the two objects, since it is required that a collision occur before the impact sensors detect any amount of impact force. Furthermore, impact sensors are unable to sense obstacles, or other cassettes in the process sinks, below the surface of process chemicals.
Another type of safety feature proposed for conventional automated wafer processing systems are strain gauges which measure the strain on the robotic arm for detecting collisions. However, this type of safety feature is also problematic. For example, differing cassette loads and different process temperatures produce a very wide and changing load to be sensed by such strain gauges, leading to inaccurate readings. Additionally, robot movements, with their accompanying acceleration and deceleration, would also compound the dynamic loading on the strain gauges making them impractical and unreliable for this purpose. Also, as with impact sensors, the use of strain gauges would not prevent collisions, since it requires a collision to initiate the obstacle detection.
U.S. Pat. No. 4,698,775 to Koch et al. teaches a self-contained mobile reprogrammable automation device for automatic transportation of particular objects from point-to-point within an enclosed environment. The device includes ultrasonic sensors placed about the transport unit to detect objects within a preselected range for collision avoidance and safety purposes. While appropriate for large obstacle sensing, ultrasonics would be inappropriate for wafer cassette detection. The semi-enclosed process equipment would create multiple echoes that would confuse the detection system. Additionally, ultrasonic detection would not detect obstacles, or other cassettes, below the surface of the process chemicals. Furthermore, if used for pausing or stopping robotic carriages or arms, the close proximity of multiple operators would cause many unneeded false stops as people would pass near the equipment.
German Patent No. DE3814582A1 to Thom et al. discloses a collision sensor for use in mobile robots and guide mechanisms of robots, wherein detection of obstacles is accomplished by touching or sensing so that collision destruction of spatially moving process units is prevented. The collision sensor employs the use of a bar magnet and a Hall element which senses the magnetic field of the magnet. The bar magnet is connected to a cylindrical sleeve which extends out from the collision sensor device. When contact is made with the extended sleeve, it shifts the position of the bar magnet within the collision sensor, thereby affecting the magnetic field. This change in the magnetic field is detected by the Hall element, which acts as a trigger or relay switch to stop the motion of the robotic device.
Although the collision sensor described in German Patent No. DE3814582A1 may be useful in preventing collision destruction, it requires that contact be made between the robot and the obstacle before the collision detection circuit is triggered. Such a feature is undesirable, particularly in applications such as semiconductor wafer processing, where even a small amount of contact between the carriers/wafers and obstacles may cause damage. Additionally, because this collision sensor uses moving parts, chemical solvents or other liquids may leak into the interior of the sensor, particularly when the sensor is submerged in a chemical bath, causing destruction of the sensor, and possibly contaminating the chemical bath. Moreover, dimensional constraints of the robotic arm and carrier do not allow for any bulky attachments. Thus, the bulky size of the collision sensor described in German Patent No. DE3814582A1 makes it impractical for use in semiconductor wafer processing systems.
It is, therefore, an object of the present invention to provide a collision detection and avoidance technique for use with automated semiconductor wafer processing systems which overcomes the disadvantages of conventional collision avoidance techniques described above. It is also an object of the present invention to provide a collision avoidance system for use in semiconductor wafer processing systems which is able to detect imminent collisions before they occur, without physical contact being made between the carrier/wafer and the obstacle. Additionally, it is an object of the present invention to provide a collision avoidance technique for use in semiconductor wafer processing systems which is able to detect the presence of other carriers within a particular processing station, even though such carriers may be submersed under liquid and undetectable using conventional collision detection techniques. Further, it is an object of the present invention to provide a low profile, non-bulky collision avoidance system which may be easily incorporated into any conventional semiconductor wafer processing system without taking up extra space in the wafer processing system.